The emergence and global dissemination of a hypervirulent pathotype of Klebsiella pneumoniae (hvKP) is concerning. In contrast to the usual healthcare-associated venue for classical K. pneumoniae (cKP) infections in the West, hvKP causes serious, life and organ threatening infections in younger, healthy individuals from the community and has the ability to metastatically spread from sites of infection. Further, what makes this variant outright scary is its potential to acquire extreme and pan-resistant antimicrobial resistance, a reality that has already begun. Few laboratories are studying hvKP and none in the US that we are aware of. Many significant knowledge gaps exist. Importantly, a reliable and efficient means to differentiate hvKP strains from cKP strains is lacking. This deficiency has resulted in under-recognition of hvKP, impeded scientific advances, and has adversely affected patient care. Identification of hvKP as the offending agent will inform the clinician to seek out occult abscesses for drainage as needed, maintain vigilance for the development of endophthalmitis (which requires immediate institution of intravitreal antibiotics & vitrectomy), optimize conditions for successful percutaneous drainage of abscesses (and thereby avoid open surgical drainage), and modify the nature and duration of systemic antimicrobial therapy as dictated by site(s) of infection. The objective of this proposal is to identify a specific genotypic and/or phenotypic marker for hvKP. Although a hypermucoviscous phenotype, as defined by a positive string test, has been used to discriminate hvKP, this test is NOT optimally sensitive or specific, especially in regions of lower hvKP prevalence. Presently, the most reliable means to identify hvKP is from the clinical syndrome of serious infection in ambulatory, healthy hosts plus/minus metastatic spread of infection. Although not all patients infected with hvKP can be identified in this manner (infection in patients with co-morbidities also occurs), using this definition will enable the generation of hvKP-rich and cKP-rich strain cohorts for comparative study. Preliminary data from our group and the literature, has identified genotypic markers (e.g. iucA (aerobactin synthesis) and 2 new potential gene markers identified by our lab) and phenotypic markers (total siderophore production and virulence in an animal infection model) that we hypothesize can be used to reliably and efficiently identify hvKP. Initial studies on hvKP-rich and cKP-rich strain derivation cohorts will identify the optimal marker, which will be subsequently assessed in independent validation cohorts. Animal data will be used to support the specificity and sensitivity of an hvKP marker that can be practically employed in both clinical microbiology and research laboratories. Data generated from this proposal will result in the development of a test for use by the clinical microbiology and research laboratories to reliably and efficiently identify hvKP. This ability will enable the needed translational advancs in the field and most importantly improved patient care.